Altair color bar chart by value not presented

问题

Trying to color a bar chart using a condition based on a value that is not presented in the chart.

I got this dataframe:

I would like to color the bar green if row.presented_value > row.coloring_value , else color red.

I saw examples of conditions by constant values and by displayed values, but couldn't make it work for me.

In the code example below I would like both foo and bar to be red.

import pandas as pd

df = pd.DataFrame({'name':['bar','foo'],
                  'presented_value':[10,20],
                  'coloring_value':[15,25]})

(alt.Chart(df, height=250, width=375).mark_bar()
 .encode(x='name', y=alt.Y('presented_value', axis=alt.Axis(orient='right')),
         color=alt.condition(alt.datum['presented_value'] > df.loc[df.name==alt.datum.x,
                                                        'coloring_value'].values[0],
        alt.value('lightgreen'),alt.value('darkred'))
        )
)

Changing the first value of coloring_value to <10 both bars will be green even though I would expect only bar to be green.

df = pd.DataFrame({'name':['bar','foo'],
                  'presented_value':[10,20],
                  'coloring_value':[5,25]})

(alt.Chart(df, height=250, width=375).mark_bar()
 .encode(x='name', y=alt.Y('presented_value', axis=alt.Axis(orient='right')),
         color=alt.condition(alt.datum['presented_value'] > df.loc[df.name==alt.datum.x,
                                                        'coloring_value'].values[0],
        alt.value('lightgreen'),alt.value('darkred'))))

Still not coloring by the correct values. Any idea on how to get it done? Thanks in advance!

回答1:

Condition expressions cannot use pandas constructs; they must map to vega expressions. Altair provides the alt.datum and alt.expr object as convenience wrappers for this.

In your case, when you want to compare two values in the row, the best way to do that is to compare them directly:

(alt.Chart(df, height=250, width=375).mark_bar()
 .encode(
    x='name',
    y=alt.Y('presented_value', axis=alt.Axis(orient='right')),
    color=alt.condition(
      alt.datum.presented_value > alt.datum.coloring_value,
      alt.value('lightgreen'),
      alt.value('darkred')
    )
  )
)

来源：https://stackoverflow.com/questions/61579368/altair-color-bar-chart-by-value-not-presented

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